Low-carbon steel sheets temper-rolled after the final anneal to improve magnetic properties

ABSTRACT

Low-carbon sheet steel having inproved magnetic properties is produced by hot rolling the steel to about 0.050 to 0.100 inch thick sheet such that the temperature thereof is 1430* to 1620*F when the steel is finished, and 900* to 1200*F when the steel is coiled. The steel is then pickled and cleaned, coldrolled to effect a thickness reduction of 40 to 80 percent, annealed to effect recrystallization, and temper-rolled to effect a plastic elongation of 6 to 10 percent.

United States Patent 1 1 1 11 3,923,560 Regitz Dec. 2, 1975 [54]LOW-CARBON STEEL SHEETS 3,188,250 6/1965 Holbein ct a]. l48/l20TEMPER.R()LLED AFTER THE FINAL gig/[gig 10x32; Carpenter et al 148/111ANNEAL To IMPROVE MAGNETIC 3:8l9:426 6/1974 Decaroetal. 148/120PROPERTIES [75] Inventor: Lester J. Regitz, Penn Township, FOREIGNPATENTS OR APPLICATIONS Westmoreland County, p [73] Assignee: UnitedStates Steel Corporation, v

Pittsburgh, Pa. OTHER PUBLICATIONS [22] Filed: 2,1975 l1-l5aZ es,5$;Trans. Asm., 25, (1937), pp. 146-147 & [21] Appl. No.: 372,432

R l s Application Data Primary Examiner-Walter R. Satterfield [63]Continuation-impart of Ser. No. 136,805, April 23, Ammey' Agen Sexton1971, abandoned.

[57] ABSTRACT [52] CL ?g '1252 7 Low-carbon sheet steel having inprovedmagnetic [51] I t Cl 2 I properties is produced by hot rolling the steelto about 58 i 22 0.050 to 0.100 inch thick sheet such that the temperale0 earc14,8...l..l..l... l g 1 1 l 1 ture thereof isl430 to 1620F whenthe steel is finished, and 900 to 1200F when the steel is coiled. 56 R fThe steel is then pickled and cleaned, coldrolled to ef- 1 e erencesfeet a thickness reduction of to percent, an- UNITED STATES PATENTSnealed to effect recrystallization, and temper-rolled to 2,242,2345/l94l Carpenter 148/3155 effect a plastic elongation of 6 to 10percent. 2,606,848 8/1952 Farling et al 1. 148/12 3,180,767 4/1965Easton et al. t. 148/120 4 Clams, 4 Drawmg Figures I5 KILOGAUSS COREL035, (WATTS PEI? POUND) I0 KILOGAUSS CORELOSS, (WATTS PEI? POUND} US.Patent Dec. 2, 1975 Sheet 1 of2 3,923,560

0.0250 IN. THICK I 0.0/85 IN. THICK I 0 l l l l l l 0 2 4 6 8 I0 I2 I4I6 I8 202224 TEMPE/P ROLL/N6 REDUCTION (/0 ELONGAT/ONI 0.0250 IN. THICK0.0/85 IN. THICK INVENTOR.

0 2 TEMPE/i ROLLING REDUCTION ELONGATIONI LESTER J. REC/T2" A fforney 46 8 l0 l2 l4 l6 I8202224 US. Patent Dec. 2, 1975 Sheet 2 of2 3,923,560

0.0/85 //V. THICK 0.0250 IN. THICK TEMPER ROLL/N6 REDUCTION (/6ELO/VCATIOW) 0.0/85 Ml. THICK 0. 0250 //V. THICK INVENTOR.

LESTER J. REG! 72 y I 1 I M Attorney 0 TEMPE/i ROLL/N0 REDUCTION,ELONGAT/O/V/ LOW-CARBON STEEL SHEETS TEMPER-ROLLED AFTER THE FINALANNEAL TO IMPROVE MAGNETIC PROPERTIES CROSS REFERENCE TO RELATEDAPPLICATIONS This is a continuation-in-part application of applicationSer. No. 136,805 filed Apr. 23, 1971, now abandoned.

BACKGROUND OF THE INVENTION Because of their superior magneticproperties, silicon sheet steels are widely used in the production ofmagnetic core components in electrical equipment such as motors,generators, transformers, and the like. These favorable magneticproperties, namely high magnetic permeability, high electricalresistance and low hysteresis losses, will minimize wasteful conversionof electrical energy into heat, and will therefore permit manufacture ofelectrical equipment having greater power and efficiency. In order toeffect and optimize the desired magnetic properties, however, thesilicon sheet steels must be produced under carefully controlled andexacting processing parameters. Silicon sheet steels are thereforesubstantially more expensive than other more conventional fiat rolledsteel products.

In the high volume manufacture of small electrical equipment forconsumer appliances, toys and the like, unit cost is perhaps the mostimportant consideration, far outweighing equipment efficiency and powerconsiderations. For these applications, therefore, electrical equipmentmanufacturers frequently utilize the less expensive, more conventionallow-carbon sheet steels for magnetic core components. Hence, there is aconsiderable market for low-carbon sheet steels having acceptablemagnetic properties for magnetic core applications.

In the course of producing low-carbon sheet steels for. magneticapplications, economic considerations have dictated that expensiveprocessing steps be avoided and that even the inexpensive steps beminimized. Therefore, even though elaborate processes have beendeveloped for producing low-carbon sheet steels having exceptionalmagnetic properties, such processes have not been adapted commercially,because the use of such processes would greatly add to the cost of theproduct, while not improving the magnetic properties of the resultantsheet to equal those of silicon sheet steels having comparable cost ofproduction. To be of any commercial value, therefore, any new processfor improving the magnetic properties of low-carbon sheet steels must beone that will not significantly increase the steels production cost.Commercially, therefore, low-carbon sheet steels for magneticapplications are produced from conventional low-carbon steel heatshaving less than 0.1 percent carbon and the usual residual elements atnormal levels for coldrolled products. The rolling procedures aresimilar to those used for other cold-rolled products. Specifically, theproduction steps are usually limited to hot rolling a low-carbon ingotto slab form; hot rolling the slab to sheet form; pickling the hotrolled sheet, cold rolling the pickled sheet for areduction of 40 to 80percent;

and annealing the sheet to effect recrystallization, generally in a boxannealing'furnace. An optional final and make it better suited forsubsequent slitting and punching operations. To optimize flatness, andhence suitability for slitting and punching, temper rolling elongationsare minimized at A to 2 percent.

The commercially produced low-carbon sheet steels for magneticapplications, when rolled to 18.5 mils thickness, typically exhibitpermeabilities in the rolled direction of from 5000 to 6000 at l0kilogauss, with core losses of from 1.3 to 1.6 watts/lb. For the samethickness at 15 kilogauss, permeabilities in the rolled directiontypically range from 2000 to 4000 with core losses of 3.0 to 4.0watts/lb. Sheets rolled to 25 mils typically exhibit permeabilities inthe rolled direction of from 4200 to 4800, with core losses of 1.8 to2.0 watts/lb. at 10 kilogauss; and permeabilities in the rolleddirection of from 2000 to 3000 with core losses of 4.2 to 4.8 watts/lb.at 15 kilogauss.

These relatively wide ranges in magnetic properties reflect anestablished tendency on the part of industry to deemphasize magneticproperties in lowcarbon sheet steel and emphasize low cost ofproduction. Nevertheless, customers have recently begun to demandimproved magnetic properties, particularly at l5kilogauss, without anappreciable increase in cost. As noted above, producers have been hardpressed to improve magnetic properties in these steels withoutsubstantial increases in production costs.

SUMMARY OF THE INVENTION This invention is predicated upon my discoverythat temper rolling the cold rolled and annealed low-carbon sheet steelbetween very critical elongation limits of from 6 to 10 percent willvery significantly enhance the steels magnetic properties to valuesnever before attained in non-silicon sheet steels. Since the otherprocess steps may be substantially the same as those presently practicedcommercially, the single modification provided by this invention, i.e.,the increased temperrolling elongation, does not significantly increasethe cost of the product.

It is an object of this invention to provide ,a new process forproducing low-carbon sheet steel having improved magnetic propertieswithout a significant increase in production costs.

It is another object of this invention to provide a new temper-rollingprocedure to be used in the manufacture of low-carbon sheet steel formagnetic applications.

It is a further object of this invention to provide a low-carbon sheetsteel having improved magnetic properties.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through 4 are graphs showingtest results of one experimental heat described :at the end of thisspecification. The graphs show permeabilities and core losses at 10 andI5 kilogauss as a function of percent temperrolling elongation.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred practice ofthis invention, the starting steel-should have a compositionsubstantially the same as those low-carbon sheet steels presentlyproduced commercially. Specifically, the composition of the steel isusually as follows: 0.02 to 0.10 percent carbon; 0.40-0.60 percent max.manganese; 0.02-0.09 percent max. phosphorus; 0.025 percent max. sulfur;

and 0.010 percent max. silicon; the balance iron and other typicalunintentional impurities.

4 art steels for two different thicknesses and tested at two differentinductions.

TABLE 1 Magnetic Properties Achieved by Inventive Process Contrasted toPrior Art To produce the sheet steel in accordance with the preferredpractice of this invention, a steel heat, having the above composition,is cast into ingot form and then hot rolled to slab form in accordancewith conventional slabbing mill practices, or continuously cast intoslab form in accordance with conventional continuous casting practice.The slab is then reheated and hot rolled to sheet having a thickness ofabout 0.06 inch or between 0.05 and 0.100 inch, such that the finishingtemperature upon exit from the finishing roll train is within the rangel430 to 1620F. The sheet is thereafter cooled with water sprays so thatit is coiled at a temperature of from 900 to 1200F. The preferred hotrolling practice is to attain a temperature within the range 1900 to1950F when the steel is about 1 inch thick, and 1460 to 1600F when thesteel is finished.

After cooling, the steel is suitably pickled to remove mill scale andthen cold rolled to effect a thickness reduction of from 40 to 80percent. Thereafter, the sheet is suitably annealed to effectrecrystallization. The annealing is preferably performed in a boxannealing furnace at a temperature of from 1 125 to 1300F for 3 to 30hours.

In some commercial operations the annealing of the cold rolled sheetsteel completes the process and the steel is sold thereafter. The mostcommon commercial practice however, has been to temper roll the annealedsheet effecting plastic elongations no greater than 2 percent for thepurpose of improving the sheets flatness and thus enhance its slittingand punching characteristics and render it more suitable for laminatedend products. Elongations in excess of 2 percent are avoided becausesuch elongations will usually result in distortions of sheet flatnessand variations in gage, or thickness, across the sheet width.

The crux of this invention resides in the unexpected discovery thatsheet steel produced in accordance with the above process can beprovided with very substantially improved magnetic properties if thefinal temper roll is sufficient to provide a plastic elongation withinthe critial range of from 6 to percent, and preferably between 7 and 9percent. As noted above, the prior art low-carbon sheet steel rolled toa thickness of 18.5 mils and tested at 10 kilogauss, typically exhibitpermeabilities of 5000 to 6000, with core losses of from 1.3 to 1.6watts/lb. In contrast thereto, sheet steels produced in accordance withthis invention will exhibit optimum permeabilities of about 7200 andcore losses of about 1.1 watts/lb. Table 1 below contrasts the optimummagnetic properties achieved by the practice of this invention with themagnetic properties obtainable in prior In considering the above table,it should of course be realized that the magnetic properties finallyachieved will vary somewhat from sample to sample even when identicalprocess parameters are employed. Nevertheless, the ranges of magneticproperties shown above for the prior art steels are the usual optimumvalues, for indeed many low-carbon sheet steels are sold for magneticapplications having magnetic properties inferior to those optimum valuesshown in Table I for prior art steels. By the same token, not all steelsprocessed in accordance with the process disclosed herein will have theoptimum magnetic properties shown in Table 1. Nevertheless, if theprocess is rigourously adhered to and there are no adverse factors toaccount for, e.g., physically damaged sheet, then magnetic propertiessuperior to the best prior art properties can be consistantly achieved.For example, at 18.5 mils and testing at 10 kilogauss, not all samplesof sheet processed according to this invention will have permeabilitiesas high as 7200 core losses as low as 1.1 watts/lb. Nevertheless, theresults should consistantly provide permeabilities in excess of 6000 andcore losses less than 1.3 watts/lb. Accordingly. even the more inferiorsamples of steel producedin accordance with this inventionwill havemagnetic properties superior to the best of the prior art steels.

The following test is presented here to illustrate the critical natureof this invention. For this test, a single heat of steel was preparedhaving the following ladle analysis:

Carbon 0.07 9? Manganese 0.57 1 Phosphorus 0.06 7( Sulfur 0.021% Silicon0.003% Copper 0.01 Z Nickel 0.01 Chromium 0.02 Molybdenum 0.01 Z Tin0.006%

This heat was cast into ingot form and hot-rolled first to slab form andthen to 0.060 inch thick sheet. Hot rolling was controlled such that thesheet was at a temperature of 1950F at 1 inch thickness. and exited thefinal rolls at 1440F. Prior to coiling, the hot rolled sheet was cooledto l 180F with water sprays.

The hot rolled sheet was then segmented into five portions, and coldrolled to various gages, such that the final or temper rolling followingannealing, various degrees of deformation could be imposed in reducingthe sheets to one of two final thicknesses. The intermediatethicknesses, final thicknesses and degree of temper rolling are shown inTable 11 below.

TABLE II Reduction Schedules and 60-Hertz Magnetic Properties 10Kilogauss Kilogauss Intermediate Percent Percent Core Loss, Core Loss.Gage, inches Elongation Reduction w/lb. Permeability w/lb. Permeability0.0185 lnch Thick Sheet 0.0188 1.6 1.5 1.52 5618 3.70 2055 0.0195 5.55.1 1.45 5629 3.07 5004 0.0200 8.0 7.8 1.12 7246 2.54 5456 0.0204 10.29.3 1.32 5905 3.02 4639 0.0222 20.0 16.7 1.48 5391 3.42 3409 0.0250 InchThick Sheet 0.0254 1.6 1.5 1.84 4761 4.62 2679 0.0263 5.0 4.9 1.83 45454.23 4054 0.0270 8.0 7.8 1.50 5155 3.79 4286 0.0275 10.0 9.1 1.84 43484.34 l 3661 0.0300 20.0 16.7 1.92 4348 4.69 3000 Following the reductionto intermediate gage, the sheets were box annealed for 12 hours at 1200Fin a nitrogen atmosphere containing 10 percent hydrogen and having adewpoint of about 70F. The sheets were then temper rolled as indicatedin the above table and sheared into test strips. The longitudinal teststrips were annealed for one hour at 1450 in the above atmosphere torelieve shearing strains, and the magnetic properties thereaftermeasured at 60 Hertz. The resulting properties are tabulated in theabove table and shown graphically in FIG 1-4, which are plots ofpermeability and core losses as a function of percent plastic elongationat 10 and 15 kilogauss. The superior effect of temper rolling between 6and 10 percent elongation is clearly demonstrated.

I claim:

1. A process for producing low-carbon sheet steel for magneticapplications consisting of; forming a steel slab consisting of 0.02 to0.10 percent carbon, 0.40 to 0.60 percent manganese, 0.02 to 0.09percent phosphorus, 0.025 maximum percent sulfur, 0.010 maximum percentsilicon and the balance iron and residual impurities; hot rolling saidslab to a thickness of 0.050 to 0.100 inch with a finishing temperaturewithin the range 1430 to 1620F; coiling the hot rolled steel at atemperature of 900 to 1200F; cooling the coiled steel to ambienttemperature; cleaning the steel to remove mill scale; cold rolling thecleaned steel to effect a thickness reduction of 40 to 80 percent;annealing the cold rolled steel at a temperature of 1 125 to 1300F toeffect recrystallization thereof; and finally temper rolling theannealed steel sufficient to effect a plastic elongation of 6 to 10percent to improve the steels magnetic properties such that at 18.5 milsthe steel will exhibit a permeability in excess of 6000 with core lossesof less than 1.3 watts/lb. when subjected to an induction of 10kilogauss, and exhibit a permeability in excess of 4000 with core lossesof less then 3.0 when subjected to an induction of 15 kilogauss; whileat 25 mils the steel will exhibit a permeability in excess of 4800 withcore losses less than 1.8 when subjected to an induction of 10 kilogaussand exhibit a permeability in excess of 3000 with core losses less than4.2 when subjected to an induction of 15 kilogauss.

2. The process of claim 1 in which said temper rolling effects a plasticelongation of 7 to 9 percent.

3. A low-carbon sheet steel for magnetic applications produced bythe'process consisting of hot rolling a steel slab consisting of 0.02 to0.10 percent carbon, 0.40 to 0.60 percent manganese, 0.02 to 0.09percent phosphorus, 0.025 maximum percent sulfur, 0.010 maximum percentsilicon and the balance iron and residual impurities to a thickness of0.050 to 0.100 inch with a finishing temperature within the range 1430to 1620F, coiling the hot rolled steel at a temperature of 900-1200F,cooling the coil, cleaning the coil to remove mill scale, cold rollingthe cleaned hot rolled steel to effect a thickness reduction of 40 topercent, annealing the cold. rolled steel to effect recrystallizationthereof, and finally temper rolling said annealed steel sufficient toeffect a plastic elongation of 6 to 10 percent, said steel characterizedby excellent magnetic properties such that at 18.5 mils the steel willexhibit a permeability in excess of 6000 with core losses of less than1.3 watts/lb. when subjected to an induction of 10 kilogauss, andexhibit a permeability in excess of 4000 with core losses of less than3.0 when subjected to an induction of 15 kilogauss; while at 25 mils thesteel will exhibit a permeability in excess of 4800 with core lossesless than 1.8 when subjected to an induction of 10 kilogauss and exhibita permeability in excess of 3000 with core losses less than 4.2 whensubjected to an induction of 15 kilogauss.

4. The steel of claim 3 in which said temper rolling effects a plasticelongation of 7 to 9 percent.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 923560 Dated December 2 19 75 Lester J. Regitz Inventor(s) It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

On the title page item [22] "Dec. 2 1975" should read June 21 1973Signed and Scaled thisthirtieth D f March 1976 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN AHPX mg Offiv Commissioner ufPaIenls andTrademarks

1. A PROCESS FOR PRODUCING LOW-CARBON SHEET STEEL FOR MAGNETICAPPLICATIONS CONSISTING OF; FORMING A STEEL SLAB CONSISTING OF 0.02 TO0.10 PERCENT CARBON, 0.40 TO 0.60 PERCENT MANGANESE, 0.02 TO 0.09PERCENT PHOSPHORUS, 0.025 MAXIMUM PERCENT SULFUR, 0.010 MAXIMUM PERCENTSILICON AND THE BALANCE IRON AND RESIDUAL IMPURITIES; HOT ROLLING SAIDSLAB TO A THICKNESS OF 0.050 TO 0.100 INCH WITH A FINISHING TEMPERATUREWITHIN THE RANGE 1430* TO 1620*F; COOLING THE COILED STEEL AT ATEMPERATURE OF 900* TO 1200:F; COOLING THE COILED STEEL TO AMBIENTTEMPERATURE; CLEANING YHE STEEL TO REMOVE MILL SCALE; COLD ROLLING THECLEANED STEEL TO EFFECT A THICKNESS REDUCTION OF 40 TO 80 PERCENT;ANNEALING THE COLD ROLLED STEEL AT A TEMPERATURE OF 1125* TO 1300*F TOEFFECT RECRYSTALLIZATION THEREOF; AND FINALLY TEMPER ROLLING THEANNEALED STEEL SUFFICIENT TO EFFECT A PLASTIC ELEONGATION OF 6 TO 10PERCENT IMPROVE THE STEEL''S MAGNETIC PROPERTIES SUCH THAT AT 18.5 MILSTHE STEEL WILL EXHIBIT A PERMEABILITY IN EXCESS OF 6000 WITH CORE LOSSESOF LESS THAN 1.3 WATTS/LB. WHEN SUBJECTED TO AN INDUCTION OF 10KILOGAUSS, AND EXHIBIT A PERMEABILITY IN EXCESS OF 4000 WITH CORE LOSSESOF LESS THEN 3.0 WHEN SUBJECTED TO AN INDUCTION OF 15 KILOGAUSS; WHILEAT 25 MILS THE STEEL WILL EXHIBIT A PERMEABILITY IN EXCESS OF 4800 WITHCORE LOSSES LESS THAN 1.8 WHEN SUBJECTED TO AN INDUCTION OF 10 KILOGAUSSAND EXHIBIT A PERMEABILITY IN EXCESS OF 3000 WITH CORE LOSSES LESS THAN4.2 WHEN SUBJECTED TO AN INDUCTION OF 15 KILOGAUSS.
 2. The process ofclaim 1 in which said temper rolling effects a plastic elongation of 7to 9 percent.
 3. A low-carbon sheet steel for magnetic applicationsproduced by the process consisting of hot rolling a steel slabconsisting of 0.02 to 0.10 percent carbon, 0.40 to 0.60 percentmanganese, 0.02 to 0.09 percent phosphorus, 0.025 maximum percentsulfur, 0.010 maximum percent silicon and the balance iron and residualimpurities to a thickness of 0.050 to 0.100 inch with a finishingtemperature within the range 1430* to 1620*F, coiling the hot rolledsteel at a temperature of 900*-1200*F, cooling the coil, cleaning thecoil to remove mill scale, cold rolling the cleaned hot rolled steel toeffect a thickness reduction of 40 to 80 percent, annealing the coldrolled steel to effect recrystallization thereof, and finally temperrolling said annealed steel sufficient to effect a plastic elongation of6 to 10 percent, said steel characterized by excellent magneticproperties such that at 18.5 mils the steel will exhibit a permeabilityin excess of 6000 with core losses of less than 1.3 watts/lb. whensubjected to an induction of 10 kilogauss, and exhibit a permeability inexcess of 4000 with core losses of less than 3.0 when subjected to aninduction of 15 kilogauss; while at 25 mils the steel will exhibit apermeability in excess of 4800 with core losses less than 1.8 whensubjected to an induction of 10 kilogauss and exhibit a permeability inexcess of 3000 with core losses less than 4.2 when subjected to aninduction of 15 kilogauss.
 4. The steel of claim 3 in which said temperrolling effects a plastic elongation of 7 to 9 percent.